1. Field of the Invention
This invention relates to photographic objectives and more particularly to photographic objectives of the rear focus type in which one or more lens components on the image side of a stop is or are movable for focusing the objective.
2. Description of the Prior Art
Color cameras having color separation prisms or other color separation optical systems such as stripe filters or mosaic filters should offer good color reproducing characteristics. To achieve this end it is generally recommended that their photographic objectives have an exit pupil at as long a distance as possible (preferably at least 200-300 mm, and ideally infinity) and that that distance be left unchanged. Even for an objective having a shortened exit pupil distance, as is sometimes necessary in actual practice, facilitating an electrical correction of the color reproduction makes it desirable that the exit pupil distance remain constant.
At the same time, the use of one of the constituent lens members which lies on the image side of the diaphragm for focusing purposes, or the employment of a so-called rear focus type of photographic objective, is advantageous for reducing the weight and the complexity of mechanical focus control signal transmission. This is so because the distance between the focusing member and the camera body is short. For this reason, a growing number of photographic objectives of this type have been developed in recent years.
As usual, the rear focus type of photographic objective is associated with a change of the position of the exit pupil resulting from the movement of the focusing member that lies behind the diaphragm. Therefore, application of this type of objective to color cameras with color separation optical systems poses a problem with respect to the color reproductivity.
This problem is also encountered when a zoom lens has its diaphragm in a space between the zoom components.
A proposal has been made to move the diaphragm of a zoom lens and one of the zoom components so as to maintain the position of the exit pupil constant. This appears in Japanese Patent Publication No. SHO 51-4135 (published Nov. 9, 1976).